HomeMy WebLinkAbout3338; Agua Hedionda & Calavera Creek Dredging; Flood Control; 1996-06-01JUFJ 2 7 1945
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for Rancho Carlsbad Mobile Home Park
Prepared for
Carlsbad Mobile
Prepared by
Howard H. Chang, Ph
June, 1996
Home Park
LD., P
P. 0. Box 9492
Rancho Santa Fe, CA 92067
TEL: (619) 756-9050
FAX: (619) 756-9460
Howard H. Chang
Consultants
Hydraulic and Hydrologic Engineering
Erosion and Sedimentation
1
. " .........
TABLE OF CONTENTS
I . INTRODUCTION ................................ ........ 1
I1 . ALTERNATIVE 1 . ON-SITE FLOODWATER DETENTIONS ..................... 2
Requirements for Floodwater Detention Basins ........................ 2
Cost Estimates .............................................. 4
I11 . ALTERNATIVE 2 . DIVERSION OF FLOODWATER FROM
AGUA HEDIONDA CREEK ......................................... 5
Preliminary Diversion Channel Geometry ............................ 6
Inletstructure .............................................. 8
Redesign of Calavera Creek Channel ............................... 8
Cost Estimate ........................................................... 9
Flow Diversion to the South ................................... 10
Cost Estimate for this Option ................................... 11 .
IV . LEVEES FOR THE CHANNEL .................................. 13
. Cost Estimate ............................................. 14
V . CONCLUSIONS AND RECOMMENDATIONS ........................ 15
Recommendations ........................................... 16
REFERENCES ................................................. 17
LISTOFFIGURES .............................................. 17
FIGURES
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FLOOD CONTROL ALTERNATIVES
FOR RANCHO CARLSBAD MOBILE HOME PARK
I. INTRODUCTION
This report is in supplement to a previous report entitled “Flooding Issues Related to Agua
Hedionda Creek for Rancho Carlsbad Mobile Home Park”. The existing channel ofAgua Hedionda
Creek through the mobile home park has been found inadequate to handle the 100-yr flood. Certain
overbank areas along the channel are within the floodplain and therefore subject to inundation. The
capacity of the existing channel was found to be the 50-yr flood. In an eflort to solve the flooding
issues, numerous consultations were held with the City of Carlsbad. Flood control alternatives were
considered and discussed in recent meetings. Pursuant to these recent meetings and discussions,
certain flood control alternatives for the Rancho Carlsbad Mobile Home Park were identified. This
report provides a study of the following flood control alternatives.
Alternative 1. On-site floodwater detention
Alternative 2. Diversion of flood flow from Agua Hedionda Creek
Alternative 3. Levees for Agua Hedionda Creek
Other measures including the advanced flood warning system and evacuation plan are being
undertaken by other consultants.
Flood hydrographs for Agua Hedionda Creek are shown in Fig. 1. The 100-yr flood has the
peak discharge of 8,080 cfs through most of the park. The flood discharge was obtained for the
ultimate development of the drainage basin for Agua Hedionda Creek. The maximum capacity of
the channel was determined by HEC-2 computations in the previous study to be 6,500 cfs, which
has the approximate return period of 50 years. Flooding duration of overbank areas for Agua
Hedionda Creek in the park is found to be about 2 hours based on the hydrograph.
The 100-yr flood hydrograph obtained from the previous hydrology study (Chang, 1989) for
Calavera Lake Creek at the Park is shown in Fig. 2; it has the peak discharge of 1,740 cfs. It is
important to point out that the hydrographs shown in Figs. 1 and 2 for these two streams have almost
the same time to peak, at the 1 lth hour.
11. ALTERNATIVE 1 - ON-SITE FLOODWATER DETENTIONS
Under this alternative, on-site floodwater detention basins will be constructed; they will be
used to store floodwater for Agua Hedionda Creek such that the 100-yr peak flood will be reduced
to be within the capacity of the existing channel in the park. The basins, of course, should be located
east of the Park to capture the flow before it enters the Park. The feasibility of constructing the
detention basin was investigated. The hydraulic requirements for such basins are provided in this
report, together with a cost estimate for construction.
The volume of floodwater is given by the area under a hydrograph. Based on the 100-yr
flood hydrograph in Fig. 1, the volume of floodwater for discharges higher than 6,500 cfs is obtained '
to be 96 acre-feet. In order to reduce the downstream peak discharge down to 6,500 cfs, the
volume of 96 acre-feet must be stored in the basins. In other words, the detention basins must
have a combined storage capacity of at least 96 acre-feet.
The golf course east of the Mobile Home Park is the logical site for such basins. It was
assumed that the present golf course grounds both north and south of the stream channel and other
undeveloped areas in the vicinity may be used as the detention basin sites. The golf course south
of the stream channel is a presently a lower area or a basin. Higher areas may also be graded to
a lower elevation. Each floodwater detention basin will be created by a berm separating the basin
from the main channel. Flows lower than 6,500 cfs would stay in the channel, but those higher
than 6,500 cfs would spill from the channel into the basin by overtopping a weir. Requirements
and evaluation of the feasibility for this scheme are described below.
Requirements for Floodwater Detention Basins - The basic requirements for the
floodwater detention basins are summarized below:
(1) They must have a combined storage capacity of at least 96 acre-feet.
(2) Water storage in the basins should be maintained at the surface elevation of 58 feet or lower
to avoid overtopping of Rancho Carlsbad Drive or flooding of adjacent areas.
(3) The bottom elevation of a basin should be higher than the low flow water surface in the
channel of Agua Hedionda Creek so that water stored in the basin may be drained into the creek
by gravity. Draining of the basin by pumping is not considered an option. Storm water in the
basin prior to the design flood may be drained if the basin level is higher than the low flow water
surface in the main channel. The stream typically has a low flow prior to the 100-yr flood. Such
a low flow may be assumed to have a water depth of 3 to 4 feet. Based on this consideration,
the bottom elevation of the basin should be kept at 53 feet or higher.
(4) Based on the requirements described above, the average depth for water storage in the basin
is 5 feet. Since the total required storage volume is 96 acre-feet, the surface area of the basins
needs to be 20 acres. This surface area will occupy all the low-land areas east of the Park to the
edge of the equestrian practice area. Fig. 3 shows the possible sites for these basins.
(5) A berm surrounding each basin is required to prevent local drainage from getting into the '
basin. Storm water runoff from adjacent hillsides should be drained into the stream directly
bypassing the basin. The berm can be made of earth; its top elevation should be 2 feet above the
adjacent ground outside the basin.
(6) A berm is also needed to separate a basin from the adjacent stream channel. Because of
limited available space, this berm will be small in width. The berm must be made to be
impervious to prevent stream water from getting into the basin by seepage. Since the berm is
quite small in thickness, its surface along the main channel must be protected with riprap to
prevent scour. Bank protection for the channel must be designed by a registered civil engineer.
Geotechnical engineer shall be consulted on the seepage protection of berm.
(7) Whenever water level in the stream channel exceeds a certain value, the excess flow will be
diverted to the basin by overflowing the weirs. Inlet structures are needed to control streamflow
into the basin. Overflow weirs will be installed at the inlet to the design elevation. The hydraulic
geometry of a weir must be designed based on detailed hydraulic computation, such that only flow
greater than 6,500 cfs will be admitted to the basin. The weirs must be of adequate size to handle
the excess discharge.
(8) Outlet structures from the basin to the stream channel shall be designed and equipped with
3
...
one-way control gates. Flap gates, such as those shown in Fig. 4, are typically used to allow
basin water draining into the stream channel but not to permit stream flow into the basin.
Cost Estimates: The cost for the requirements described above are itemized below.
(1) Earth moving and disposal: Excavation of the basins requires earth moving of 180,000 cubic
yards. The earth removed from the basins will be disposed at nearby sites to be selected. The
unit cost depends on the location of disposal sites; it is assumed to be $ 1.20 per cubic yard for
excavation and disposal.
Costforitem ............................................. $216,000
(2) Berms surrounding basins: The total cost is estimated to be $20,000.
Cost for item .............................................. $40,000
(3) Bank protection for berms adjacent to channel: These berms will have a total length of about
2,300 feet along both banks of the channel for Agua Hedionda Creek. Bank protection will have
the length of 2,300 feet, 2 to 1 side slope, height of 14 feet including toe entrenchment, surface
width of 28 feet, and surface area of 64,400 square feet. The unit cost is assumed to be $ 7 per
square foot.
Cost for item ............................................. $450,800
(4) Inlet structure ............................................ $60,000
(5) Outlet structure and flap gates ................................. $ 15,000
(6) Golf course reconstruction ................................. $ 1,400,000
(8) Loss of residents activities for 9 months ........................... $ 80,000
(7) Loss of income during golf course reconstruction for 9 months ........... $ 350,000
(9) Permit fees: Permits for the project may be required from the City, County, Army Corps of
Engineers, U. S. Fish and Wildlife, California Fish and Games, etc .......... $ 150,000
Subtotal ................................................ $2,761,800
Continency: 10 % of the combined cost ............................ $276,180
Total for this alternative ..................................... $3,037,980
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111. ALTERNATIVE 2 - DIVERSION OF FLOODWATER FROM AGUA HEDIONDA
CREEK
Agua Hedionda Creek at the Park has the capacity only for the 50-yr flood. An idea
suggested by the City of Carlsbad for flood control is to divert the part of flood discharge beyond
the channel capacity of Agua Hedionda Creek to the north to Calavera Lake Creek or to the south
across El Camino Real directly to the lagoon. These schemes are now investigated; their details
are described below. It should be noted that the diversion to the north alternative is contingent
upon the completion of the proposed Calavera Lake Creek channel improvement.
A diversion channel from Agua Hedionda Creek to Calavera Lake Creek is expected to
be on the east side of the mobile home park. A possible alignment for the diversion channel is
shown in Fig. 5. In order to handle the required discharge of diversion, this channel must have
sufficient width, depth, and slope. Since the channel must also fit in the existing physical
constraints of topography and land use, it became necessary to investigate the feasibility as the
first step. The purpose of the feasibility study is to determine if a diversion channel with adequate
capacity can be constructed within the given physical constraints. If it is found feasible, then
hydraulic geometries of the channel and a cost estimate shall be provided in the study.
This diversion will increase the flow of Calavera Lake Creek, therefore this creek must
be enlarged to handle the augmented flow. It is the assumption that sedimentation problems for
Calavera Lake Creek will be considered as a part of the future channel design. Sediment
investigation for Agua Hedionda Creek and Calavera Lake Creek is not a part of the present
scope.
It should be pointed out that a detailed hydraulic analysis for the channel system will
involve the use of the HEC-2 program that is outside the present scope. In such an analysis for
the channel system including Agua Hedionda Creek, the diversion channel, and Calavera Creek,
different sets of channel geometries shall be used for the diversion channel, the intake, and
Calavera Creek channel. Physical constraints shall be applied. These channel geometries shall
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be adjusted in order to arrive at the adequate geometries, it possible. The detailed analysis will
also involve hydrologic simulations to determine the modified hydrographs. The diversion will
change the flood discharges for Agua Hedionda Creek and Calavera Lake Creek. Hydrologic
simulations shall be made for flood routing from Agua Hedionda Creek to Calavera Lake Creek.
Since the times for floods to peak for these creeks are somewhat different due to the different
drainage basin areas, the timing for these flood hydrographs shall be considered in flood routing
in arriving at the modified hydrographs as a result of diversion.
Preliminary Diversion Channel Geometry - For this feasibility study, the effort is to
arrive at preliminary estimates of the hydraulic geometry such that a preliminary cost estimate can
be made at this time. In the following steps, the hydraulic geometry of the diversion channel as
shown in Fig. 6 is first obtained.
(1) Channel length: For the channel alignment along the eastern boundary of the Park, the route
from Agua Hedionda Creek to Calavera Creek has a length of about 2,400 feet. This means that
the diversion channel shall have the length of 2,400 feet.
(2) Channel capacity; The diversion channel must have the minimum capacity of 1,600 cfs. This
value is the 100-yr discharge of 8,080 cfs subtracted by 6,500 cfs which is the channel capacity
for Agua Hedionda Creek. To allow a safety margin, the diversion channel should have the
design capacity of 2,000 cfs.
(3) Permissible velocity: An earth channel with natural vegetation is recommended for the
diversion channel. This type of channel is considered more desirable environmentally. In order
to maintain channel stability, the maximum velocity should be kept at 5 feet per second or lower
to avoid significant potential scour. In other words, the velocity of 5 feet per second is the
permissible velocity.
(4) Channel cross-sectional area: The cross-sectional area is the channel capacity divided by the
permissible velocity, Le.,
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A = 2,00015 = 400 square feet.
(5) Channel depth: The diversion channel should have a depth no greater than 10 feet in order to
be compatible with Agua Hedionda Creek. With a freeboard of 2 feet, the flow depth is 8 feet
in the channel.
(6) Side slope: The channel should have a side slope of roughly 2.5 to 1 to maintain stability for
the earthen bank material.
(7) channel roughness: with natural vegetation, the roughness coefficient in terms of Manning’s
n is estimated to be 0.04.
(8) Bed width: Based on the conditions described above, the bed width of the channel is computed
to be 30 feet. Its water-surface width is 70 feet, and the channel width is 80 feet at the top.
These dimensions provides a cross-sectional area of flow of 400 square feet. The wetted perimeter
of the channel is 73.1 feet, and its hydraulic radius is 5.47 feet.
(9) channel slope: The channel slope is computed using the Manning formula based on the
conditions given above.
S = n*~*/(2.21xR~’~)
= 0.04* x 5*/(2.21~5.47~’~)
= 0.00188
(10) Required energy head: The required energy head includes the drop in water surface elevation
along the diversion channel from Agua Hedionda Creek to its outlet at Calavera Lake Creek. The
head difference is the channel slope times the channel length. Additional head is required at the
inlet, estimated to be 0.5 feet.
Head = 2,400 x 0.00188 + 0.5 = 5 feet
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,
The terrain is rather flat along the route of the diversion channel. It is possible to achieve the 5
feet of energy head. However, this will require a lowering and a redesign of the Calavera Creek
channel.
Inlet Structure - The flow admitted into the diversion channel must be controlled. The
inlet structure, as shown schematically in Fig. 7, is used to control the inflow to the diversion
channel. This structure may be a guide vane that extends into the main channel to split the total
flow into two parts. The flow split must be carefully designed such that the diversion channel
carries the flow in excess of the channel capacity for Agua Hedionda Creek. The inlet structure
may also be a weir. The shape of the weir must be carefully designed to achieve the object of
flow control. Control gates should be avoided if possible.
Redesign of Calavera Creek Channel - A preliminary design of the Calavera Creek
channel was made some 8 years ago by P&D Consultants. The designer was Mr. Dicken Hall.
The previous design assumed a channel alignment outside the fence of the Mobile Home Park.
As a result of the flow diversion, the hydraulic geometry for Calavera Lake Creek must
be adjusted to handle the augmented flow discharge. This channel must also meet Agua Hedionda
Creek at the existing confluence just upstream of the El Camino Real Bridge. The existing bed
elevation at the confluence is the downstream control point for Calavera Lake Creek. Any
adjustments in channel-bed slope shall still meet this base elevation.
(1) Change in flood discharge - With a part of the flow diverted from Agua Hedionda Creek, the
flood discharge of Calavera Lake Creek will be augmented. A comprehensive hydrology analysis
is required to route the flow from Agua Hedionda Creek into Calavera Creek. The routing study
will produce the new flood discharge while considering the timing of these flow events. Flood
hydrographs for these two channel as shown in Figs. 1 and 2 indicate that the peak discharges of
these two streams occur at nearly the same time. As a rough estimate, the increase in discharge
for Calavera Lake Creek may be computed by adding the diverted flow of 2,000 cfs to the 100-yr
discharge of 1,750. Thus, the new 100-yr discharge will be 3,750 cfs for Calavera Lake Creek
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after the confluence of the diversion channel.
(2) Change in cross-sectional area - Under the previous design, the improved Calavera Channel
would be an environmental channel which maintains a low velocity for the sake of channel
stability. With a new discharge more than twice the original value, the cross-sectional area of the
channel will be increased by 1.14 times. Other channel dimensions must be adjusted by the
designer to accommodate the additional flow.
(3) Change in channel slope - The Calavera Lake Creek channel is the downstream control for the
diversion channel. In order to provide the required energy head for the diversion channel, the
slope of the Calavera Lake Creek channel must be adjusted. Any change in slope will also
involve changes in other channel dimensions.
(4) Feasibility for channel modifications: The slope of the terrain along the Calavera Lake Creek
has a drop in elevation of more than 10 feet from El Camino Real to the northeastern edge of the
Park. This slope is considered adequate for the future channel with the required modifications.
Cost Estimate - Preliminary cost estimates for this diversion alternative are provided herein.
The cost estimate in this study reflects costs for materials and labor, engineering design, and right-of-
way acquisition. The material and labor cost was based upon construction costs for local projects.
The itemized costs are given in the following:
(1) Right of way: The diversion channel will have a surface area of 5.5 acres. The decrease in land
value due to channel construction is estimated to be $ 38,000 per acre.
Cost for item ......................................................... $209,000
(2) Diversion channel: Construction of the channel will involve excavation of
50,000 cubic yards, which costs $1.2 per cubic yard. Cost for item ................ $60,000
(3) Inlet structure: ....................................................... $50,000
(4) Additional cost for Calavera Lake Creek channel .......................... $250,000
(5) Channel maintenance at $3000 per year for 100 years ........................ $300,000
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channel, a few existing mobile home lots will be sacrificed.
(4) Bridges at Don Ricardo and Don Miguel: These bridges are required on the diversion channel
in the Park.
(5) Underpass at El Camino Real: Double 10’ x 7‘ box culverts with a length of 120 feet are
considered adequate. There exist several utility lines under El Camino Real, including those for
gas, electricity, sewer, water, telephone, and cable. It will be necessary to raise or to lower these
utility lines to make room for the culverts.
(6) Concrete-lined channel south of El Camino Real: A concrete-lined trapezoidal channel along
the south edge of El Camino Real is a part of the diversion channel. This channel has the bed
width of 11 feet, side slope of 1 to 1, channel depth of 8 feet, and surface width of 27 feet. This
channel is 1,600 feet long.
(7) Energy dissipator at downstream exit of diversion channel: The diversion channel exits into
Agua Hedionda Creek south of El Camino Real. The exit is near the proposed Cannon Road
Bridge. Because of the high flow velocity in the diversion channel, an energy dissipator is needed
at the exit.
(8) Feasibility: It is hydraulically feasible to construct the diversion channel in consideration of
the total energy head available. The drop in terrain elevation from the entrance of the diversion
to its outlet is about 8 feet. This total head is sufficient for the concrete-lined channel described
above. However, the concrete-lined channel needs to be kept clean to maintain its function.
Cost Estimate for this Option - Cost for required items are listed below:
(1) Elimination of existing lots at Park: Lots 53 and 87 will be eliminated to make room for the
channel. The lot price is $85,000 per lot. Cost for item . . . . . . . . . . . . . . . . $ 170,000
(2) Right of way acquisition south of El Camino Real: 1.5 acres at $25,000
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per acre ................................................. $500 37.
(3) Inlet structure .......................................... $000 90.
(4) Concrete channel in Park: Excavation of 3. 900 cubic yards at $ 1.2 per cubic
yard yields $4. 680. Concrete bed 11. 000 ft2 at $4 per ft2 gives $44. 000. Retaining
walls 8. 800 ft2 at $ 9.5 per square foot yields $ 83.600 . Cost for item ........ $280 132.
(5) Box culverts: 120 feet long at $ 720 per foot ...................... $400 86.
(6) Road work for El Camino Real: 4. 800 sqaure feet at $3 per square feet ..... $400 14.
(7) Excavation for culvert placement: 1. 250 cubic yards at $1.2 per cubic yard ... $ 1. 500
(8) Concrete channel south of El Camino Real: Excavation of 16. 600 cubic yards at $1.2 per cubic
yard yields $ 19. 200. 54. 400 square feet of concrete at $2.85 per square foot gives $ 155.040 .
Cost for item ............................................. $174. 240
(9) Outlet structure: ......................................... $50. 000
(10) Levee for main channel ..................................... $000 10.
(11) Bridges at Dom Ricardo Dr . and Don Miguel Dr ..................... $85. 000
(12) Relocation of utility lines under El Camino Real ............... Over $000 1.000.
(13) Permit fees: Permits for the project may be required from the City. County. Army Corps of
Engineers. U . S . Fish and Wildlife. California Fish and Games. etc .......... $000 150.
Subtotal ........................................... Over $2.001. 320
Contingency: 10 % of subtotal .................................. $200. 132
Total for this alternative ................................. Over $2.201. 452
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IV. LEVEES FOR THE CHANNEL
Under this alternative, levees will be constructed along the banks of Agua Hedionda Creek
through the Park to contain the 100-yr flood. An analysis has been made for this alternative.
Hydraulic requirements for levees have been obtained and provided. The effectiveness and
impacts of this alternative on flood control have been analyzed. A cost estimate is also presented.
While the levees keep the channel water away from the park land, it must allow the
drainage of local storm runoff into the channel. This function can be served by using flap gates
installed on the levees that allow one-way flow into the channel. Peak flow in the channel lags
behind the local runoff in timing because of the difference in the time of concentration. Therefore,
local runoff may be drained through the flap gates before the channel peaks.
Levees will prevent flooding of the park land by keeping floodwater in the channel.
However, this alternative for flood control has the following considerations:
(1) Encroachment of channel flow by levees will raise the flow velocity in the channel. While
such raises may be small, they, nevertheless, increase the channel instability. For this reason, the
levees may become a legal liability. It is therefore advisable to obtain the consent of residents
before proceeding with this alternative.
(2) The levees are not used to improve the stability of the flood channel. In fact, the levees are
subject to flood damages if its foundation is scoured. Prevention of potential scour will involve
extensive construction that is outside the scope of the present study.
(3) Presence of levees causes view obstruction which may be considered undesirable
environmentally.
Now, required features of the levees are described below.
(1) Length of levees - Levees are needed along the channel reach where the overbank areas are
subject to flooding by the 100-yr flood. Based on this criterion, the levees should start from the
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wooden bridge at Rancho Carlsbad Drive and ends at El Camino Real for a total channel length
of about 2,500 feet.
(2) Height of levees - The 100-yr water-surface profile for the channel was computed and
presented in the pervious study. Construction of levees at the top of the channel bank will
encroach on the width of the floodplain by excluding overbank areas from the channel. Such an
encroachment will raise the water-surface elevation. The top of the levee should be the 100-yr
flood level plus a freeboard of about 2 feet. Based on this criterion, the levee height varies from
2 feet to 5 feet, with the average being about 3.5 feet.
(3) Toe of levees - During the design flood, the levees will be under water pressure from the
channel side. The toe entrenchment must be sufficient to resist the turning moment due to
hydrostatic pressure. The average toe entrenchment is estimated to be about 3 feet to protect it
against turning.
(4) Control gates - The levees must be equipped with one-way flow control gates. Such control
devices are needed to drain water from the park area into the channel.
(5) Water proof - The levees must be impermeable.
Cost Estimate - Cost for this alternative include the costs for levees, control gates, etc.
(1) Levees, 32,500 square feet, at $7.00 per sqaure foot ................. $227,500
(2) Flap gates .............................................. $5,000
(3) Decrease in property value, 15 % decrease for 42 lots valued
at $85,000 per lot .......................................... $535,500
(4) Permit fees: Permits for the project may be required from the City, County, Army Corps of
Engineers, U. S. Fish and Wildlife, California Fish and Games, etc .......... $ 300,000
Subtotal ................................................ $1,068,000
Contingency at 10 % of subtotal ................................. $ 106,800
Total for this alternative ..................................... $ 1,174,800
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V. CONCLUSIONS AND RECOMMENDATIONS
This report provides analyses and cost estimates for three flood control alternatives for the
Rancho Carlsbad Mobile Park. The Park has a total area of 154 acres; it is located in the drainage
basin of Agua Hedionda Creek that is 17.3 square miles in area at this point. The Park
constitutes 1.4 % of the total drainage basin area for the creek. There have been several flood
control studies for the Park in recent years. The fundamental problem stems from the fact the
channel has the capacity for the 50-yr flood which no longer satisfies the present standard for
flood control in the County of San Diego. Since a part of the park land is subject flooding by the
100-yr flood, flood control has thus become a concern. It is also recognized that the duration of
flooding for overbank areas of the channel is about 2 hours during the 100-yr flood. Potential
damages due to overbank flooding has been found to be limited in value. The existing channel
is partially protected with bank protection. While erosion potential for the channel still exists,
erosion-related damages by floods may not be accurately estimated.
. This study does not cover Calavera Lake Creek, for which a flood control measure has
already been adopted by the City of Carlsbad. Under this measure, Calavera Lake Creek will be
improved to convey the 100-yr flood. Implementation of this measure will benefit the mobile
home park.
Alternative 1 for flood control is to use floodwater detention basins for flood discharge
reduction. Alternative 2 is to reduce the discharge of Agua Hedionda Creek by diverting a part
of flow away. It should be noted that the diversion to the north alternative is contingent upon the
completion of the proposed Calavera Lake Creek channel improvement.
Damages to the park due to flooding has been estimated by the Park manager to be $5,000
for each occurrence. In a 100-yr time span, the occurrence of flooding is also the frequency that
a 50-yr storm is exceeded. In other words, two flooding occurrences can be expected in the 100-
yr time span. With flood control, flooding damages will be eliminated and thus the benefits due
to flood control is $ 10,000 in a 100-yr time span.
The costs for the alternatives are summarized below. It is clear that the cost for each
alternative is much higher than the flood control benefits it provides. The benefit-cost ratios for
these alternatives may not justify the undertaking.
Alternative
1. Detention basins
Total Cost Benefit-Cost Ratio
$ 3,037,980 0.0033
II ~~
2a. Diversion to the north I $ 1,120,900 I 0.0089
3. Levees
2b. Diversion to the south I Over $ 2,201,452 I Less thanOOr11
$ 1,174,800 0.0085
Alternative 3 has a lower cost. However, flow encroachment by levees will raise the flow
velocity thereby causing channel instability, which may become a legal liability. The levees are
subject to damages if its foundation is scoured.
All three alternatives considered in this study involve the use of structures for flood
control; they are classified as structural flood control measures. These alternatives have been
found to be undesirable for different reasons. The alternative to the structural measures is the
non-structural flood control measures which involve no structural construction of any kind along
Agua Hedionda Creek. Such measures included the following:
(1) Advanced flood warning
(2) Evacuation
(3) Monitoring and maintenance of flood channel.
These plans are being developed by others.
Recommendations: On the basis of the study, the flood control alternatives are ranked
in consideration of the cost, desirability, and other factors. The ranking order is as follows:
First choice: Non-structural flood control but with advanced flood warning and evacuation plans.
Second choice: Alternative 2a - Diversion to the north.
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Third choice: Alternative 3 - Levees. While this alternative is the less expensive, it is necessary
to obtain the consent of the residents who should be informed of the consequences described in
this report.
Not recommended: Alternative 1 (floodwater detention basins) and Alternative 2b (diversion to
the south) are not recommended.
REFERENCES
1. Chang, H. H., 1989, "Hydrological Study for Northeastern Carlsbad".
LIST OF FIGURES
Fig. 1. Hydrographs of 100-yr flood for Agua Hedionda Creek in the Park
Fig. 2. Hydrograph of 100-yr flood for Calavera Lake Creek
Fig. 3. Location of on-site floodwater detention basins
Fig. 4. Picture of flap gates
Fig. 5. Alignments of diversion channel
Fig. 6. Cross-sectional profile of north diversion channel
Fig. 7. Schematic of the inlet structure for diversion channel
17
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m rl
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=$ .3
0
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m
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cr
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 N 0 mwcr N 0 m (D cr N 3 rl rl rl 4
Fig. 4. Picture of flap gates
I - 80 feet -4
.. _. Fig. 6. Cross-sectional profile of diversion channel ..- ... .. : .. .. ..
Fig. 7. Schematic of the inlet structure for diversion channel